(1) Field of the Invention
The present invention relates to an amine catalyst for the preparation of a fine-cell rigid polyurethane foam. More particularly, the present invention relates to a process for the preparation of a rigid polyurethane foam having a fine-cell structure wherein a polyol containing a blowing agent and other additives is reacted with a polyisocyanate in the presence of the amine catalyst.
(2) Description of the Related Art
A rigid polyurethane foam is generally prepared by an instant mixing of a polyol containing a blowing agent such as Freon and water and a silicon foam stabilizer with a polyisocyanate, while stirring to effect blowing. The rigid polyurethane foam has a light weight and an excellent heat-insulating property, and therefore, the polyurethane foam is widely used in fields where heat insulation for maintaining a high temperature or a low temperature is necessary, for example, for construction material, boards, electric refrigerators, freezers, plants and the like.
The rigid polyurethane foam is urethane resin having closed cells. Each closed cell contains in the interior hereof Freon gas having a low thermal conductivity, a relatively small amount of carbon dioxide gas formed by reaction between the isocyanate and water at the formation of the foam, and air. This closed-cell structure imparts a high heat-insulating property to the rigid polyurethane foam. The heat-insulating property of the rigid polyurethane foam is generally expressed by the thermal conductivity called the "K-factor value", and a similar K-factor value indicates a higher heat-insulating property.
In view of the compression strength of the rigid polyurethane foam as a construction material, to reduce this K-factor value, the cell size must be small, the closed cell ratio must be high, the Freon concentration in the cells must be high and the foam density must be low.
With recent advances in research into a rigid polyurethane formulation, and in foam-forming techniques a "fine-cell rigid polyurethane technique" has been developed according to which the quantity of Freon as the blowing agent is increased while the amount of water in the formulation is reduced, and the reaction rate of the system is greatly increased, with the result that the cell size of the foam is made finer and the K-factor value is drastically reduced. Since the K-factor value can be drastically reduced by this fine-cell rigid polyurethane technique, compared with the level of the conventional technique, in fields where a heat-insulating material having a high performance, for example, for electric refrigerators, reduction of the wall thickness of a heat-insulating polyurethane material can be realized and a high energy saving effect and large capacity of refrigerators through space saving effect can be obtained by this technique.
However, the extremely high reaction rate and the reduction of the amount incorporated of water in the above-mentioned fine-cell rigid polyurethane foam system bring various disadvantages. For example, since the reaction rate of this system is extremely high, sufficient time is not allowed for mixing the polyol and isocyanate, and since the time for casting the reaction liquid is not sufficient, a reaction starts in the already cast reaction liquid and there is no substantial flowability of the reaction liquid. Accordingly, the flowability of foam is poor at the blowing reaction and an impingement face (share line) is formed between two adjacent foams, with the result that a problem of unevenness of the foam density arises. In the case of cast blowing in the mold, since the blowing reaction abruptly occurs, removal of air from a vent hole at a rate corresponding to the blowing rate becomes difficult. This is another problem. According to the conventional technique, to overcome these disadvantages, for example, at the cast blowing step in an electric refrigerator, a plurality of casting heads are adopted for casting the starting liquid, and in an extreme case, a method is adopted in which the starting liquid is cast from five casting heads. Special equipment is necessary for smoothing the removal of air from the vent hole to maintain a constant air pressure according to the blowing reaction rate of the system and not disturb the flow of foams in the system and, for example, a forced evacuation system is adopted. This drastic change of the blowing equipment or adoption of a complicated control system in the manufacturing process requires a large equipment investment and raises a barrier to a stable manufacture of high-quality products.
A reduction of the amount incorporated of water in the rigid polyurethane foam formulation results in a decrease of urea bonds formed by reaction of the isocyanate with water in the urethane-forming reaction. Accordingly, the compression strength of the foam is reduced and the dimensional stability of the foam, especially the dimensional stability at low temperatures, is degraded. Therefore, a high foam density becomes necessary for compensating the reduction of the compression strength due to the decrease of urea bonds and the above-mentioned unevenness of the density distribution, and for compensating the degradation of the dimensional stability at low temperatures. Since this high foam density results in a retardation of the curing rate of the polyurethane foam, a problem arises of an impossibility of obtaining an elevation of the manufacturing speed. As apparent from the foregoing description, the conventional fine-cell rigid polyurethane foam technique involves problems that are solved from the economical and manufacturing viewpoints.
Japanese Unexamined Patent Publication No. 54-130697, Japanese Examined Patent Publication No. 57-56491, and Japanese Unexamined Patent Publication No. 60-58418 teach that an organic carboxylic acid salt of a tertiary amine compound, such as a formic or octylic acid salt, can be used as an amine catalyst having a retarding effect for the polyurethane-forming reaction, and the foam moldability, curing speed and processability can be improved. But in these patent publications, the problems involved in the preparation of fine-cell rigid foams having a foam density of 20 to 40 kg/cm.sup.3 and a cell diameter smaller than 250 .mu.m are not discussed, and it is not suggested that the use of formic acid as an assistant and a catalyst comprising specific tertiary amine compound is valuable for solving these problems.